Purine nucleoside phosphorylase (PNP) catalyses the phosphorolytic cleavage of the ribo- and deoxyribonucleosides of guanine and hypoxanthine to give the corresponding sugar-1-phosphate and guanine or hypoxanthine.
Humans deficient in purine nucleoside phosphorylase (PNP) suffer a specific T-cell immunodeficiency due to an accumulation of dGTP and its toxicity to stimulated T lymphocytes. Because of this, inhibitors against PNP are immunosuppressive, and are active against T-cell malignancies. Clinical trials are now in progress using 9-(3-pyridylmethyl)-9-deazaguanine in topical form against psoriasis and in oral form for T-cell lymphoma and immunosuppression (BioCryst Pharmaceuticals, Inc). The compound has an IC.sub.50 of 35 nM for the enzyme. In animal studies, a 50 mg/kg oral dose is required for activity in a contact sensitivity ear swelling assay in mice. For human doses, this would mean approximately 3.5 grams for a 70 kg human. With this inhibitor, PNP is difficult to Inhibit due to the relatively high activity of the enzyme in blood and mammalian tissues.
Nucleoside hydrolases catalyse the hydrolysis of nucleosides. These enzymes are not found in mammals but are required for nucleoside salvage in some protozoan parasites. Some protozoan parasites use nucleoside phosphorylases instead or as well for this purpose. Inhibitors of nucleoside hydrolases and phosphorylases can be expected to interfere with the metabolism of the parasite and therefore be usefully employed against protozoan parasites.
It is an object of the invention to provide pharmaceuticals which are very effective inhibitors of PNP and/or nucleoside hydrolases.